In experimental fluid mechanics, a visualization of flow patterns using a laser-based split-beam method is a conventional technique for obtaining a qualitative and/or quantitative analysis of the flow patterns. In an exemplary manner, a laser beam is fanned out into an approximately two-dimensional light plane, thereby illuminating a liquid or gas flow and the particles contained therein. Fanning out the beam to form a light section with a maximum thickness of just a few millimeters (up to 2 or 3 mm), which can thus be viewed more or less as a two-dimensional light plane, can be achieved, e.g., using an optical deflection unit with a rotating mirror or using a cylindrical lens. The particles added to the flow medium emit scattered light upon passage of the light segment.
The particles themselves must be able to follow the flow with as little drift as possible and should vary only minimally during the measurement. The laser light scatter triggered by the particles being carried in the flow can be detected by the naked eye, using video equipment, photography, or by other means. Both qualitative and quantitative analyses of the optical refraction and scatter phenomena can be carried out. A quantitative analysis is based on illumination of the flow field on the light section plane, using a continuous wave laser. In principle, exposure time t over the luminosity period of the laser can be set. With a known exposure time t and a known object-to-image ratio a of the camera or the optical system, distance s or flow velocity v can be measured by analyzing particle trail a.multidot.s on the image, using the relation v=a.multidot.s/t.
A particle image velocimetry (PIV) method is a conventional method, in which a pulsed laser is used to double-expose the flow field. A determination of a.multidot.s is provided in a similar manner from the distance between points in associated pairs of points and from the pulse interval.
Quantitative analyses of the images, however, require object-to-image ratio a to be first calibrated. The relevant geometric, optical and recording parameters must either be known or determined through measurements. Calibrations of this type or corrections to influencing recording parameters are relatively complicated and must be either redetermined or calculated each time an individual parameter changes.